What does this double stack mean?

It means rather than having your ammo all in a neat one on top of the other line the ammo is the mag is staggered into two somewhat loose stacks allowing for more ammo capacity. Look at a 1911 mag and a Glock or other higher cal mags. It's pretty easy to tell he difference. A double stack mag it wider and some smaller hands have issues with a good or as comfortable grip on them due to that.